As a fixing apparatus for use in an electrophotographic image forming apparatus such as a copying machine or a printer, a heat roller type fixing apparatus is widely used. In general, the heat roller type fixing apparatus includes a pair of rollers (a fixing roller and a pressure roller) which are pressed against each other. This type of fixing apparatus includes heating means (made of a halogen heater, for example) provided inside both or either one of the pair of rollers. This type of fixing apparatus fixes a toner image in the following manner: The heating means heats the pair of rollers to a predetermined temperature (fixing temperature). After that, a recording sheet on which an unfixed toner image is formed is fed to a pressure area (fixing nip area) between the pair of rollers, and then the recording sheet is caused to pass through the pressure area. Thus, the toner image is fixed on the recording sheet due to heat and pressure applied thereto.
Incidentally, a fixing apparatus included in a color image forming apparatus generally uses an elastic roller. The elastic roller is a fixing roller provided with, on its surface, an elastic layer which is made of e.g., silicon rubber. In the case where the elastic roller is used as the fixing roller, a surface of the fixing roller elastically deforms according to an uneven surface of an unfixed toner image, and is in contact with the unfixed toner image so as to cover the unfixed toner image. This allows a color unfixed toner image whose toner amount is larger than that of a monochrome unfixed toner image to be favorably fixed. Further, due to strain release of the elastic layer which occurs in a fixing nip area, it is possible to improve releasability of the fixing roller with respect to color toner, which is more likely to offset than monochrome toner. Furthermore, since the elastic layer of the fixing roller can be depressed, the fixing nip area has a nip shape protruding upward (i.e., toward the fixing roller side), that is, a so-called inverse nip shape. This makes it possible to more favorably separate a sheet from the fixing roller, thereby allowing the sheet to be separated without using any separation means such as a separation claw (self stripping). This eliminates a problem of an image defect which is caused by the separation means.
However, in such a fixing apparatus provided in a color image forming apparatus, increasing a process speed (a traveling speed of a sheet in a fixing nip area; a fixing speed) may cause insufficient heat transfer to an unfixed toner image and thereby lead to improper fixing of the unfixed toner image. In view of this, in order that the process speed is increased while sufficient heat is supplied to the unfixed toner image and the fixing process is carried out properly, it is necessary to increase the nip width of the fixing nip area. As means for increasing the nip width, (1) a method of increasing the thickness of the elastic layer of the fixing roller or (2) a method of increasing the diameter of the fixing roller is considered.
However, since the elastic layer has a very low heat conductivity, the fixing roller which is provided with the elastic layer and internally includes the heating means as in the conventional configuration has such a problem that a surface temperature of the fixing roller cannot respond to a temperature change of the heating means when the process speed is increased. Thus, in the case where the thickness of the elastic layer is increased as in the above-mentioned method (1), the problem with the temperature response of the surface of the fixing roller becomes more serious. In addition, due to the low heat conductivity of the elastic layer, the method (1) also causes problems of: an extended warm-up period for heating; and an increased power consumption.
In the case where the diameter of the fixing roller is increased as in the method (2), the size of the fixing roller is increased and thereby its heat capacity is increased. This causes problems of: an extended warm-up period for heating; and an increased power consumption.
In view of this, as a technique for solving these problems, such a technique (external heat fixing method) is proposed that external heating means is in contact with a surface of a fixing roller so that the fixing roller is heated from the outside.
For example, Patent Literature 1 discloses an external belt heat type fixing apparatus including: a fixing member; an endless belt (external heating belt) suspended by a plurality of suspending rollers; heating means for heating the endless belt, the endless belt being pressed against the fixing member so that the fixing member is heated.
Patent Literature 2 discloses a belt fixing type fixing apparatus including a fixing roller; a heating roller; a fixing belt which is endless and is suspended between an outer peripheral surface of the fixing roller and an outer peripheral surface of the heating roller; and a pressure roller for pressing the fixing roller via the fixing belt, wherein a fixing process is carried out in such a manner that a recording sheet on which an unfixed toner image is formed is fed to a pressure area (fixing nip area) between the fixing belt and the pressure roller and is caused to pass through the pressure area. This belt type fixing method also adopts a configuration in which the fixing belt heated by the heating roller is carried to the surface (outer surface) of the fixing roller so that the fixing roller is heated from the outside. In view of this, this method is also considered to be a kind of the external heat fixing method.
In these fixing apparatuses adopting the external heat fixing method, the belt having a small heat capacity is used to heat the fixing roller from the outside. This makes it possible to heat the fixing roller quickly, thereby leading to a shorter warm-up period. Consequently, it is possible to prevent the problems such as the impaired temperature response of the fixing roller and the extended warm-up period, and also to secure a wide nip width by providing a thick, low-hardness elastic layer to the fixing roller and/or increasing the diameter of the fixing roller, for example.